Temperature compensation



ELECTROMAGNETIC RESPONSIVE DEV/CE Inventor Frank W Mew-Til I,

His Attorney.

F. W. MERRILL TEMPERATURE COMPENSATION Filed May 11, 1935 IlfIlIiflI/Iffflfff/ll ill!!! llllllrvllitlllllflllflllllllltll' June 8,1937.

NEGATIVE TEMPERA'TURE CO FFICIENT 0 RES/STANCE iatented June 8, 1937UNITED STATES TEMPERATURE CODIPENSATION Frank W. Merrill, Fort Wayne,!nd., assignor to General Electric Company, a corporation of New YorkApplication May 11, 1935, Serial No. 21,061

Claims. (Cl. 175-335) My invention relates to temperature compensationand more particularly to resistors used for the temperature compensationof electro-magnetically operated electro-responsive devices.

5 Most electro-magnetically operated electroresponsive devices, such forexample as meters, instruments, relays and regulators are provided withan operating winding or windings of copper wire. Copper has anappreciable positive temperature coeflicient of resistance which meansthat changes in ambient temperature change the electrical resistance ofthe. windings thereby changing the effective calibration or setting ofthe device. One way to compensate for this is to connect in series withthe copper winding a resistor of the proper value and having the propernegative temperature coefiicient of resistance. Theoretically, as thetemperature changes the increase in resistance of the copper Windingwill just be offset by the decrease in resistance of the compensatingresistor so that there is no net change in the resistance of the circuitas a result of the temperature change. I have found, however, that suchresistors have such a relatively small surface area that theself-heating in the resistor seriously affects its value as atemperature compensator. By selfheating, I mean the heating produced inthe resistor as a result of the current flow therethrough, asdistinguished from the heating of the resistor as the result of anincrease in ambient temperature. Thus, due to the relatively smallsurface of the resistor, relatively high temperatures are attained bythe self-heating before this heat can be radiated and a heat balanceobtained.

Obviously, therefore, the temperature of the compensating resistor doesnot follow the ambient temperature so that its change in resistance doesnot complement the change in resistance of the copper winding.

In accordance with my invention I overcome this difficulty by in effect,greatly increasing the heat radiating surface of the resistor while at 5the same time preventing direct radiation from the surface of theresistor and also preventing heat loss due to convection currents at thesurface of the resistor. A preferred way of doing this is to embed thecompensating resistor in a thermally conducting electrically insulatingmedium such for example, as dry sand. If desired, the sand may beconfined by a metal container in the center of which is placed thecompensating resistor so that the whole forms a sub-' stantially unitaryassembly.

My invention is equally applicable to temperature compensating resistorshaving temperature coefficients which are either less than and greaterthan zero, that is to say, either positive or negative coeflicients.With zero temperature coeflicient resistors there is no need for my in--vention as the resistance is independent of the temperature. Suchresistors are some times used for rough or partial temperaturecompensation. In such cases their action may be described as dilutingthe over-all temperature coeflicient of the circuit.

An object of my invention is to provide a new and improved temperaturecompensating resistor assembly.

My invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

Referring now to the single figure of the accompanying drawing, which isa diagrammatic showing of an embodiment of my invention, I is anelectric circuit to which is connected an electro-magnetically operatedelectro-responsive device 2, of any type, which is provided with anoperating winding 3 composed of a conductor having either a positive ora negative temperature coefficient of resistance. In practically allcases the winding will be made of copper having a positive temperaturecoeflicient of resistance and this temperature coefficient of thecompensator will be negative as indicated. If the device 2 is a voltageregulator employed to regulate the voltage of an automobile batterycharging generator, (which it may be assumed to be) the winding 3 willbe subjected to temperatures varying from 30 degrees below zero F. todegrees F., under the hood of the automobile. Such a wide temperaturerange would produce a resistance change in the copper winding whichwould vary the setting of the regulator by an amount which could not betolerated.

For'compensating for this temperature error I connects in series withthe winding 3 a negative temperature coeflicient of resistance resistor4 which may be of any suitable type, such for example as a rod of thecarborundum mixture which is known to the art as "Globar. The ends ofthis rod are coated with metal at 5 so as to make them highly conductingand connected or clamped thereto are electrical terminals 6 which areinsulated by suitable insulating means I from a metal casing or cylinder8 which may be constructed of steel. Filling the space in the cylinder 8not occupied by the resistor 4 and terminals 6 is any suitable thermallyconducting electrically insulating material, such for example as aquantity of dry sand 9. This assembly has been shown on the drawing onan exaggeratedly large scale so as more clearly to illustrate itsdetails.

With the above construction it is impossible to have any convection aircurrents at the surface of the resistor l and also it is impossible tohave any heat radiation from this surface and all the heat istransferred to the sand by conduction and the heat loss in the assemblyis practically entirely due to radiation from the surface of thecylinder 8. This surface is so much larger than the surface ofresistance 4 that the self-produced heat can be radiated at lowtemperature. As a result, the temperature of the'rod more nearlyreflects the change in the ambient temperature surrounding the cylinderthan it would if the rod were simply exposed to the air.

Another advantage of this construction is that the thermal capacity ofthe device is increased so that the cylinder as a whole will heat upslowly at a rate comparable to that of the heating of the coil 3.

A further advantage of this construction is that the thermal capacity ofthe device may be so adjusted by a choice of the size of the externaltube 8, and the thickness of the thermally conducting, electricallyinsulating medium 9, as to bring the temperature of the compensatingresistance due to self heating to just the right point so that thedecrease in resistance of the rod 4 P will be closely equal to theincrease in resistance of the coil 3 due to its self heating.

For these two reasons the voltage calibration of the circuit as a wholewill be unaffected by the -self heating during the preliminary warmingup of the coil and the compensating rod and both coil and rod will atall times respond slowly to changes in the ambient temperature atapproximately the same rate and in proportionally corresponding amountsto maintain constant circuit resistance and thus insure propertemperature compensation of the device under all conditions.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that changesand modifications can be made without departing from my invention and I,therefore, aim in the appended claims to cover all such changes andmodiflca tions as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A temperature error compensating arrangement of the type in which aresistor having a temperature coeflicient of resistance other than zerois connected to compensate a winding having a temperature coefflcientother than zero of an electro-responsive device for errors produced byvariations in ambient temperature characterized by means for minimizingerrors in said compensation produced by self heating in said resistorcomprising heat conducting material for increasing the heat radiatingarea of said resistor to a point where the temperature of said resistorfollows substantially the ambient temperature.

2. An ambient temperature compensating resistor assembly comprising, incombination, a rod of resistance material having a negative temperaturecoeiiicient of resistance, and means for making the temperature of saidrod follow closely the ambient temperature of said assembly comprising ametal container surrounding said rod, and a quantity of dry sand fillingthe space in said container not occupied by said rod.

3. A temperature error compensating element for an electro-responsivedevice having an operating magnet with a winding having a positivetemperature coefllcient of resistance comprising a negative temperaturecoeflicient resistor adapted to be connected in series with said windingfor compensating said device for ambient temperature changes, and aquantity of thermally conducting electrically insulating material placedaround said resistor for causing its temperature to be relativelyindependent of its own heating eifect.

4. A temperature error compensating element for an automobile voltageregulator having a copper operating winding whose resistance changeswithchanges in ambient temperature adversely aifects the calibration of saidregulator comprising a rod of resistance material having a negativetemperature coeflicient of resistancesuch as to eliminate said adverseeffect adapted to be connected in series with said winding,a metalcylinder axially surrounding said rod, and a quantity of dry sandfilling the space between the outer surface of said rod and the innersurface of said cylinder.

5. An ambient temperaturecompensating device for a voltage regulatorwhich has a copper operating coil comprising, in combination, a rod ofresistance material which has a negative temperature coeflicient ofresistance adapted to be connected in series with said winding, metalcoating on the ends of said rod, electric terminals connected to saidcoating, a metal cylinder axially surrounding said rod and terminals,and dry sand filling the space in said cylinder not occupied by said rodand terminals.

FRANK W. MERRILL.

